Water is often used as a heat-transfer medium because it affords high rates of heat transfer at relatively small expense and it is thermally stable and non toxic. However, water has definite limitations to its use as a heat transfer fluid. Water is only useful over a limited temperature range and has a tendency to corrode metals that it comes in contact with.
Glycol based heat transfer fluids (with or without corrosion inhibitors) are often added to aqueous systems to broaden the operable temperature range. As a result of its low cost, ethylene glycol is the most commonly used head transfer fluid base. Due to its low boiling point, however, ethylene glycol is not practically useful for high temperature applications.
Aqueous glycol-based heat transfer fluids generally create problems with respect to metal corrosion. Anions such as bicarbonates, sulfates and chlorides, which have a corrosive effect on the metals they contact, may be present in the water medium. The glycol base itself when subjected to conditions of elevated temperatures and aeration often develops corrosive tendencies towards metals. To combat corrosion, heat transfer fluids typically contain corrosion inhibitor additives capable of buffering the solution to a pH between 7 and 11.
The prior art describes glycol-based heat transfer compositions. For example, U.S. Pat. No. 2,147,395 discloses liquid compositions with non-corrosive and anti-freeze properties comprising an alcohol, and a mixture of a soluble nitrate and a soluble salt of tungstic, selenic or molybdic acids. Examples of alcohols disclosed by this patent include the monohydroxy alcohols and polyhydroxy alcohols such as ethylene glycol, propylene glycol, diethylene glycol, mixtures of such glycols with glycerol. This patent further discloses that a soluble phosphate can optionally be added to the composition to further protect aluminum.
U.S. Pat. No. 2,147,409 discloses liquid compositions with non-corrosive and anti-freeze properties comprising an alcohol and a soluble salt of tungstic, selenic or molybdic acid. Ethylene glycol, propylene glycol, diethylene glycol and glycerol are listed as useful alcohols. This patent further discloses that 0.1% of a soluble phosphate can be added for further protection of aluminum if desirable.
U.S. Pat. No. 2,384,553 discloses an ethylene glycol-based antifreeze containing an inhibitor combination of a glycol soluble inorganic borate and phosphate salts.
U.S. Pat. No. 3,030,308 discloses single phase antifreeze compositions and aqueous solutions thereof comprising a water soluble liquid freezing point depressant, magnesium metaborate and an alkali metal molybdate. This patent discloses that the freezing point depressants comprise water miscible liquid alcohols such as monohydroxy lower alkyl alcohols and liquid polyhydroxy alcohols such as alkylene and dialkylene glycols. Specific alcohols disclosed by this patent are methanol, ethanol, propanol, isopropanol, butanol, ethylene glycol, diethylene glycol, propylene glycol, butylene glycol and mixtures thereof. This patent further discloses that the preferred glycol is ethylene glycol which can contain about 10% diethylene glycol.
U.S. Pat. No. 4,389,371 discloses an anti-freeze composition comprising (a) an alcohol selected from at least one of the group consisting of methanol, ethanol, propanol, ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol and glycerol and (b) a corrosion inhibitor comprising an effective amount of a conventional corrosion inhibitor selected from the group consisting of an alkali metal carbonate, borax, an alkali metal dichromate, an alkali metal silicate, phosphorous acid, phosphoric acid, an alkali metal tungstate, benzotriazole, tolyltriazole, an alkali metal salt of benzoic or toluic acid, an alkali metal salt of a phenol, an alkanolamine and an organo-silicone compound in addition to an alkanolamine phosphate and a heterocyclic nitrogen-containing compound. This patent further discloses that ethylene glycol is the preferred alcohol.
U.S. Pat. No. 4,452,758 discloses an antifreeze concentrate composition comprising a water-soluble alcohol selected from the group consisting of methanol, ethanol, propanol, ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol and glycerol, and alkanolamine, an alkali metal phosphate, an imidazoline and a conventional corrosion inhibitor.
U.S. Pat. No. 4,455,248 discloses a single phase glycol based antifreeze composition comprising one or more glycols selected from the group consisting of ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol and glycerol and additionally comprising water, sodium metasilicate, phosphate of potassium, sodium metaborate or sodium tetraborate, sodium nitrate, a copper corrosion inhibitor and alkali. This patent further discloses that a phosphate of potassium is necessary for the purposes of the composition and that dipotassium phosphate (K.sub.2 HPO.sub.4) is preferred.
U.S. Pat. No. 4,545,925 discloses a freeze protected, heat transfer fluid composition comprising at least about 0.25 weight percent phosphate as PO.sub.4, greater than 0.025 weight percent nitrite as NO.sub.2, and the balance a polyhydroxyalcohol such as propylene glycol, diethylene glycol and, preferably, ethylene glycol.
U.S. Pat. No. 4,548,787 discloses that water-soluble salts of an acid selected from the group consisting of molybdic, tungstic, and selenic acids in combination with a water-soluble phosphate provide a synergistic improvement in the retardation of corrosion of aluminum water pumps in contact with aqueous liquids such as aqueous antifreeze compositions containing a water soluble alcohol freezing point depressant.
Representative of some commercially available ethylene glycol based heat transfer fluids are Ambitrol CN.RTM. and Dowtherm.RTM. SR-1 available from Dow Chemical Company, JEFFCOOL.RTM. E-100 available from Texaco, and UCARTHERY.RTM. available from Union Carbide Corporation.
However, the known glycol based compositions frequently possess characteristics that render the compositions undesirable for use as higher temperature heat transfer fluids. For example, ethylene glycol and many corrosion inhibitors commonly used in antifreeze formulations, e.g. alkanolamines and some hetercyclic nitroqen compounds, can thermally degrade at high operating temperatures (see, e.g., Herndon, L. R. and Reid, E. E., Decomposition of Organic Compounds at High Operating Temperatures and Pressures, J.A.C.S(50), p. 3066-3073 (1928)). This degradation shortens the operating life of the fluid and requires replacement of the fluid. Frequent replacement of the fluid in industrial applications is costly and often involves time consuming system shutdowns.
Additionally, certain additives useful in the prior art glycol based compositions, such as phosphate-containing corrosion inhibitors, are not soluble in concentrated triethylene glycol solutions. Consequently, these additives can form undesirable precipitates in heat transfer fluid concentrates containing triethylene glycol.
Accordingly, it is the purpose of this invention to provide single phase triethylene glycol-based heat transfer fluid concentrates and aqueous compositions thereof which are useful over a broad range of temperatures; which are thermally stable over the operable temperature range, particularly at high temperatures; and which provide corrosion protection.